1. Field of the Invention
The present invention relates generally to the field of ultrasonic fish detection. More specifically, it relates to a directional ultrasonic Doppler apparatus for detecting actively swimming fish or the impulsive movements of fish responding to a fishing lure or other stimulus introduced into the aquatic environment by a fisherman.
In the past, a wide variety of ultrasonic systems, generally called sonar, have been used for the detection and localization of fish. Usually, pulse echo techniques have been used wherein repetitive pulses of ultrasonic energy are transmitted in a narrow beam into a body of water and the presence of, and distance to, fish or the like are indicated by the amplitude and time delay characteristics of the returning echo. Other sonar systems have employed the continuous transmission of ultrasonic energy, and the detection of fish depends on the frequency changes of the return signal. Some such systems have utilized a continuous transmission Doppler technique. The present invention specifically relates to an improved ultrasonic Doppler apparatus for detecting the movements of fish including, but not limited to, the responses of fish to the presence of an artificially introduced fishing lure.
2. Description of the Prior Art
In the past, two Doppler fish detectors analogous to the present invention were patented and manufactured. Initially, a highly directional Doppler fish detector using an ultrasonic frequency of 800 kHz was manufactured based on U.S. Pat. No. 3,123,798 (Holloway); and later another directional Doppler fish detector, integral with a radio receiver, using an ultrasonic frequency of 1 mHz was manufactured based on U.S. Pat. No. 3,231,852 (Kritz). Although they were both marketed aggressively, these products encountered unforeseen operation problems when used under typical fishing conditions, and the manufacture and sale ended after a very brief period. Since then, almost 20 years ago, no such product has been manufactured or marketed.
The directional Doppler fish detectors of the prior art were both characterized by the use of a relatively high ultrasonic frequency, so that even slowly moving fish would produce an audible (rather than a sub-sonic) Doppler signal. However, this sensitivity to slow movements eventually proved to be a serious detriment under practical fishing conditions. Very strong Doppler signals from the bottom resulted from even relatively slow boat drift (e.g., a 300 Hz Doppler signal was caused by drifting in a 0.5 knot current). Furthermore, surface waves also returned a strong Doppler shifted signal when the beam was directed near the horizontal (e.g., Doppler signals up to around 500 Hz would typically result from even moderate wave action). The consequences of such relative movements, as they pertain to the prior art, are shown graphically by the line 37 in FIG. 5.
However, even more serious problems encountered by the prior art were caused by the very poor transmission characteristics of high frequency ultrasound in scattering and absorbing suspensions such as commonly exist in most lakes and streams. Very high ultrasonic signal attenuation was caused by a combination of: (1) viscous absorption in the water itself of almost 20 dB for a 100 meter (2 way) path, (2) absorption by particulate suspensions which typically resulted in an additional attenuation of 20 to 40 dB under moderately turbid conditions, (3) Rayleigh scattering loss from microbubbles caused by wave action, and (4) resonant scattering loss from microbubbles caused by wave action. The relative absorption and scattering losses of the "high" frequency ultrasound used in the prior art and the "low" frequency ultrasound used in the present invention are compared and discussed in detail with reference to FIGS. 9 and 10.
The object of the present invention is to overcome the serious limitations of the high frequency Doppler fish detectors of the prior art which were found to be virtually inoperative when used in aquatic environments containing even moderate concentrations of particulate suspensions. Since such suspensions commonly occur in most lakes and streams, the manufacture and use of such Doppler fish detectors was discontinued almost 20 years ago. Since then, a need has existed for a low cost, horizontally directed fish detector for bait casting and certain other modes of sport fishing.